Inductive device



March 7, 1939. P. s. DARNELL INDUCTIVE DEVICE Filed Sept. 30, 1937 3 Sheets-Sheet l INVENTOR By R S. DARNELL A 7' TORNE Y March 7, 1939. P s, DARNELL INDUCTIVE DEVICE Filed Sept. 50, 1937 5 Sheets-Sheet 2 FIG. 4

lNVENTOR RSDARNELL BY AT TORNEV Patented Mar. 7, 1939 UNITED STATES PATENT OFFICE Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application September 30, 1937, Serial No. 166,514

.19Ciaima.

This invention relates to system for the transmission of intelligence and more particularly to inductive devices or coils having adjustable magnetic material cores and fixed windings associated r, therewith which are used in such systems.

In the art of the transmission of intelligence by electric currents one of the features which may impair the quality of the transmission is the interference caused in one circuit due to currents il' in another adjacent circuit. An extreme case probably occurs in transmission by carrier currents applied to cables where the individual pairs constituting the circuits are close together. This interference between adjacent circuits has been is given the general term cross-talk and as used in this specification the term cross-talk will inlines to aid in balancing out cross-talk voltages in said lines.

A further object is an inductive device having a plurality of fixed windings and adjustable cores of magnetic material so arranged that the mutual inductance between the sets of windings entering into the circuit pairs may be varied while the inductive reactance of these sets of windings remains substantially constant.

These and other related objects will be apparent and the invention will be better understood from the following description and drawings forming a part thereof, iii-which Fig. 1 is a perspective drawing of the inductive device of this invention with a part of the cover removed;

Fig. 2 is a longitudinal section of the winding spool showing the windings and adjustable cores; Fig. 3 is a section of the spool. etc., taken on line 3-3 of Fig. 2;

Fig. 4 is a wiring diagram showing in detail the windings on the spool;

Fig. 5 is a schematic showing how the windings are related when the device is inserted in two cable pairs;

device adapted for incorporation in transmission Figs. 6 and 7 are curves which will be used in explaining the operation of the device; and

Figs. 8, 9 and 10 illustrate where capacities exist between turns of the device and how such capacities are balanced out.

A number of suggestions have been made relative to cross-talk elimination based upon balancing out the interfering voltages. Reference is here made to two patents which have issued in this connection: Coolidge et a1. Patent 2,008,061 10 which discloses a transmission system designed for cross-talk elimination and providing a scheme of cross connections between cable pairs which may be extended as more pairs of the cable areutilized for carrier transmission and Weaver Patent 2,080,217 which is an improvement over the system of Coolidge et al. This present invention is a further improvement over the Weaver patent in that it is directed to an improved inductive device which may be incorporated in the system of :0 Weaver.

The inductive device of this invention as illustrated in Figs. 1, 2, 3 and 4 comprises a hollow tube ii of insulating material on which windings are mounted in two sections l2 and ii. The tube 5 on which the windings are mounted may be of any satisfactory insulating material; in practice phenol fibre tubing has been found satisfactory.

Within the tube H are two cylindrical magnetic material cores I4 and I5 fixedly mounted on a so non-magnetic material rod it. The cores are preferably of molybdenum permalloy dust in accordance with U. S. Patent 1,768,443 to Elmen. This dust may be prepared in accordance with U. S. Patent 1,669,649 to .Beath et a1. and ground as to a sumcient fineness to pass through a 400-mesh sieve. After the proper degree of fineness is attained the particles are insulated and pressed into the cylindrical forms.

The non-magnetic rod I 8 is preferably brass 40 supported at one end in an insulating washer II. The other end of rod I6 is screw-threaded through a metallic bushing I! which is inserted in insulating head is. Rod i6 is equipped with a detachable hexagonal head 20 whereby the rod may be turned to adjust the position of cores i4 and i5 relative to the windings l2 and I3. It is understood that the hexagonal head may be replaced by any convenient means of turning rod I.

The terminals 2| of the windings are inserted in holes which extend through insulating head I! to facilitate connection to the windings. These terminals are staked to the head near the outside surface and are of such a length that they may be bent to contact with and be soldered to terminals of an adjacent coil when required, thereby making unnecessary the use of separate strapping wires between units disposed as indicated in Weaver Patent 2,080,217.

On tube H near head 19 are mounted two auxiliary spools 22 and 23 on which are mounted two inductive resistance windings 24 and 25 which are connected in shunt to certain of the main windings for a purpose which will appear later.

An aluminum or other light metal non-magnetic shield or container 26 fits over a reduced portion of head l9 to form an enclosure for the tube 1 i, windings, etc. Wax or other insulating compound is introduced into the container to prevent the entrance of moisture.

,The windings are mounted on tube It in two sections of two layers each and each layer of each section contains two individual windings or coils. These individual windings are so wound that the wires cross only once per turn. This winding arrangement will be seen more particularly in Fig. 4 wherein block II represents the tube or winding spool, l4 and 15 the cores and IS the rod upon which the cores are mounted. I Before proceeding with a description of the manner of -winding shown in Fig. 4 it may be noted from the circuit diagram of Fig. that section l3 comprises four separate windings or coils 34, 36, 38 and 40; and section l2 comprises four separate windings or coils 35, 31, 39 and 4|. Coils 34, 31, 3B and.39 are wound in one direction and coils 35, 36, 40 and 4| are wound in the opposite direction. Still considering both Figs. 4 and 5 and as viewed from the left of the figures, coil 34 consists of alternate turns of the inner layer of section l3 wound in a counter-clockwise direction, while coil 36 consists of the remaining turns of the inner layer of section l3 but wound in aclockwise direction. Coil 38 consists of alternate turns of the outer layer of section l3 wound in a counter-clockwise direction, while coil 40 consists of the remaining turns of this outer layer wound in a clockwise direction. Similarly, in the other section coil 35 consists of alternate turns of the inner layer of section i2 wound in a clockwise direction and coil 31 consists of the remaining turns of the inner layer wound in a counterclockwise direction. Coil 39 comprises alternate turns of the outer layer of section I! wound in a counter-clockwise direction and coil 4| comprises the remaining turns of the outer layer wound in a clockwise direction.

Starting from terminal I and following wire R it will be seen that this wire is wound counterclockwise viewed-from the. left-hand end of the spool and constitutes alternate turns of the inner layer. From terminal I it proceeds to the right, then up and behind the spool to the lower edge, then up and diagonally to the right and again around the spool for the required number of turns in this section. The wire W is wound clockwise (viewed from the left) and proceeds from terminal 2 to the right, then down and behind the spool, then up and to the front and then diagonally to the right and again around the spool as required.

Wires R and W then pass to the second section i2 in a groove corresponding to groove 21 (Fig. 1) out in the diametrically opposite side of tube or spool ll. These wires are twisted an odd number of times in passing from section 83 to section i2 and are wound in reverse direction in section i2 to that of section i3. That is,'wire R is wound in clockwise direction as viewed from the left end while wire W is wound in a counterclockwise direction. The method of winding is the same in section l2 as in section l3 and is such that the wires of each pair cross but once' for each turn. Wires R and W have a typical point of crossing at point 26 in section I3 and. at point 29 in section l2. Wire R is brought out to terminal 5 and wire W is brought out to terminal 6.

In practice the wires of one pair are wound one turn at a time to give only the single crossing. For example, wire R is given one turn in one direction and then wire W is crossed over wire R and given one turn next to wire R but in the opposite direction. Then wire R crosses over wire W as at 28 and again wire R is given another turn next to wire W. This process is continued until windings 34 and 36 are completed.

Wires G and B from terminals 3 and 4 are placed in the second layer but start on the side of tube ll opposite the point at which wires R and W start and cross at the completion of each turn'as at typical point 30 in section l3 outer layer and at typical point 3| in section I2. The crossing points are of. course on the opposite side of tube II to the crossing points of wires R and W. In passing from section l3 to section l2 in groove 21, wires G and B retain their same relative positions, that is, they are twisted an even number of times, and are wound in the same direction in section l2 as in section l3.. Wires G and B are brought out to terminals .1 and 8, respectively.

Inductive resistance 24 is shunted across the windings of wire R between terminals l and 5 while inductive resistance 25 is shunted across the windings of wire-W between terminals 2 and 6. 1

The manner in which this unit operates will be seen more clearly from Fig. 5 which shows two cable pairs of a transmission system. Each pair constitutes a circuit over which communication may be had.

To describe the operation of the device it will be assumed that signals are being transmitted over the pair comprising conductors i5 and 2-6 of Fig. 5 and that due to conditions elsewhere in the circuit between the two pairs involved there is induced in the pair comprising conductors 3-1 and 48 a voltage which arises from the signal current'in the pair of whichconductors l-5 and 26 are parts. This induced voltage will tend to cause current to flow in the pair of which conductors 3-4 and 48 are parts and this current would give rise to disturbance in transmission in the latter pair.

The device operates as follows to neutralize this voltage; At a given instant assume that current is flowing in conductor i-5 from I to 5 and in conductor 2-43 from 6 to 2, as indicated by the dotted arrows. The magnetic flux set up in the individual coils will then be as shown by the solid arrows. The flux due to coil 36 will be from right to left as viewed in the'drawings. Since the winding of coil 35 is reversed relative to that of coil 3 3 the flux wiii also be reversed. In coil 3? current is passing from right to left and since the direction of winding is reversed relative to that of coil 35 the flux in coils 35 and 31 will be in the same direction. For a similar reason the flux in coil 36 will be in the opposite direction to that of coil 31 but inthe same direction as that of coil 36.

Coil 35 is spaced at such a distance from cell 55 and coil 35 is spaced at such a distance from coil 31 that there is substantially no threading of the flux due to coils 34 plus 38 with coils plus31, nor of the flux due tocoils 35 plus 31 with coils 34 plus 38; that is, substantially no mutual inductance exists therebetween. The flux due to coils 34 and 38 will thread coils 38 and 48 in the direction of the solid arrow associated with coils 38 and 48 since the latter coils are superimposed on the former. Also, the fiux due to coils35 and'31 will thread coils 39 and 4| in the direction shown by the solid arrow associated with coils 39 and 4|. The conditions as just recited exist when the movable cores are in their midpositions, as indicated in Figs. 2, 4 and 5.

The cross-talk voltages induced in the pair with which conductors 3-1 and 48 are associated due to conditions external of the device and due to the signal current present in the cablepair associated with wires |"5 and 28 may be in a direction to cause current to flow in the same direction as the signal current in the pair with which conductors I5 and 28 are associated or they may be in the opposite direction.

Assume, first, that these voltages are in a direction which would cause current to flow in the same direction as the signal currents, that is, the induced voltages external of the device are such as to tend to cause current to fiow from 3 to 1 and from 8 to 4. Since coil 34 is wound in the same direction as coil 38 and coil 38 is wound in the same direction as coil 48 and the fiux due to coils 34 and 38 threads coils 38 and 48 in the same direction as, that of coils 34 and 38, the induced voltages in coils 38 and 48 will be in a direction to oppose those set up externally of the devise. vice coils 35 and 31 are wound in opposite directions and the flux due to coils 35 and 31 threads coils 39 and 4| in the same direction. The voltages induced in coils 39 and 4| due to the currents in coils 35 and 31 are such as to aid the voltages set up externally of the device. To neutralize the cross-talk voltages the coupling between coils 34 and 38 and coils 38 and 48 must be increased, while the coupling between coils 35 and 31 and coils 39 and 4| must be decreased. This is accomplished by moving cores 4 and I5 from their mid position toward the left, as shown in Fig. 5; that is, the cores must be moved out of the field of coils 35, 31, 39 and 4| and into the field of coils 34, 38, 38 and 48. The opposing flux is therefore increased in accordance with the assumed conditions while the aiding flux, is decreased. Tests of the pair including conductors 3-1 and 4-8 will indicate when the cores have been moved a sufiicient distance to neutralize the undesired voltages.

If the voltages induced externally of the device are in the opposite direction to that assumed in the preceding discussion, then the voltages due to .coils 35 and 31 must dominate and this is accomplished by moving cores l4 and I5 to the right, as shown in Fig. 5; that is, the cores must be moved out of the field of coils 34, 38, 38 and 48 and into the field of coils 35, 31, 39 and 4|.

Fig. 6 shows by means of a curve the manner in which the total mutual inductance between the pair |5 and 2--8 and the pair 3-1 and 48 reverses as the cores are moved. The ordinates of the curve 'are mutual inductances in microhenries measured with a current having a frequency of 1 kilccycle while the abscissae in- In the other section of the de-' dicate positions of the core in thirty-seconds of an inch to the right or left of the mid position. When the cores are in the mid position indicated at position zero, the net or resultant mutual inductance of the device is zero because the mutual inductance in section i2 is equal and opposite to the mutual inductance in section I3.

As the cores are moved from the mid position the resultant mutual inductance changes. This resultant mutual inductance as the cores are moved toward the left is indicated as positive. Therefore, the resultant mutual inductance when the cores are moved to the right will be negative, as will be readily understood from the previous discussion in which it was shown that the flux which dominates changes asthe cores move through the mid position. The cores may therefore be adjusted as to position to new tralize the cross-talk voltages no matter which direction they tend to cause current to flow and no matter what their magnitudes within the limits of operation of the device. It is readily seen from this curve that the absolute or numerical values of mutual inductance are substantially equal for an equal movement of the cores ineither direction from the mid position.

Fig.7 illustrates graphically how, as the mutual inductance in sections l2 and I3 varies, the inductive reactance in the pair composed of con-, ductors |-5 and 2-8 remains substantially constant. In this figure, the ordinates are inductances in microhenries measured with a current having a frequency of 1 kilocycle while the abscissae indicate the positions of the cores in thirty-seconds of an inch to the right or left of the mid position. The effective inductance of coils 34 plus 38 is shown in curve E This curve is the same type of curve as that of Fig. 6 except that curve E shows the variation in total inductance, that is mutual plus self, for coils 34 plus 38, whereas the curve of Fig. 6 shows the variation in the resultant mutual inductance only of all the coils. Curve E shows that with the cores in the mid position the total inductance of coils 34 and 38 is about 2.45 microhenries and that as the cores move to the left the total inductance reaches a peak of about 3.20 microhenries. When the cores are moved to the right the total inductance of coils 34 plus 38 falls to about 1.20 microhenries. Similarly, curve F shows the variation in total inductance of coils 35 plus 31 with movement of the cores.

The maximum and minimum values for coils 35 plus 31 are the same as for coils 34 plus 38, the minimum for curve F occurring at the position of the cores for the maximum of curve E and the maximum for curve F occurring at the position of the cores for the minimum of curve E.- At the mid position of the cores, the total inductance of coils 34 plus 38 is equal to the total inductance of coils 35 plus 31.

Curve G of Fig. 7 shows the measured variation in total inductance of the loop |5-8-2 which includes coils 34, 35, 38 and 31 with change in position of the cores for the full eflective movement of the cores. It will be noted that this curve is substantially fiat.

Curve H of Fig. '7 shows the variation in total inductance of loop |-5-82 as determined from curves E and F, that is, curve H is derived from curves E and F by adding the ordinates of E and F. Curve H is given for purposes of comparison to show that the shape of the curve of the measured values is substantially the same as the derived values. The difference in absolute values of curves G and H is due to a slight mutual inductance which exists between the coils of section I2 and the coils of section l3. To separate sections l2 and I3 far enough to eliminate this slight mutual inductance would make the device too long for a desirable design. This small amount of mutual inductance is insufficient to interfere with the efllcient operation of the device in its practical application.

Curves E, F and G show that as the total inductance-of coils 34 and 36 increases with the cores moving to the left the total inductance of coils 35 and 31 decreases and that theamount of increase is substantially equal to the amount of decrease. Also, since these coils are wound with wire of uniform dimensions and characteristics and since the diameters of the coils are equal, the increase of inductance of coil 34 equals the decrease of inductance of coil 35; hence, the total inductance of coil 34 plus that of coil 35 remains substantially constant. This is also true with respect to coils 36 and 31. Therefore, the inductive reactance of line I5 is equal to. that of 2-6 and this inductive reactance remains substantially constant with varying mutual inductance due to the movement of the cores.

The conditionsdiscussed with reference to loop l-5-6-2 are also true of loop 3-184. There is, therefore, no inductive unbalance in the event of longitudinal currents and no crosstalk resultant therefrom. Longitudinal currents may occur due to noise or like interference. If power current to energize repeaters is transmitted over the signal cable it probably will be impressed as longitudinal currents, that is, the pair comprising conductors l5 and 2-6 may constitute one power conductor while another pair, such as 3-1 and 4-8, may constitute the other conduc- .to'r. With the device of this invention adjusted for neutralizing side-to-side cross-talkthere will therefore be no cross-talk due to longitudinal currents which might be considered a type of phantom-to-side cross-talk.

As noted earlier in the Specification, inductive resistance winding 24 is connected in shunt to coils 34 and 35 and inductive resistance winding 25 is connected in shunt to coils 36 and 31. The purpose of these inductive resistance windings is to make the mutual inductance characteristic of the coils 34 to 4|, inclusive, simulate the mutual inductance existing between any two cable pairs. Representing the mutual inductance of coils 34 to 4 I, inclusive, by mr and the mutual inductance existing between cable pairs by me, theoretical conditions which have been checked by measurements show that me, due to proximity effect, is complex in character and may be represented by the formula ma+jmb,- where me is the in-phase component and jmb is the component 90 degrees out of phase as indicated byj which is etc., are likely to give rise to cross-talk because of the capacities whichexist between the wires of the coils. This is well understood by those.

skilled in the art and efforts are made in design ing such coils to maintain what is known as sideto-side capacitance .unbalance at a minimum since the greater this unbalance the greater is the cross-talk arising from this type of unbalance.

Figs. 8, 9 and 10 illustrate how in the present device this type of cross-talk is kept at a minimum by reducing the side-to-side capacity unbalance to substantially zero. In Fig. 8, the relation of the individual turns of the sections of the device are shown. In section l3 the first turn of wire G is shown as being directly over the first turn of wire R in accordance with the wiring diagram of Fig.4. The first turn of wire B is over the first .turn of wire W and next to the first turn of wire G, while the first turn of wire W is next to the first turn of wire R. The second turn of wire G is next to the first turn of wire B and is over the second turn of wire R which is next to the first turn of wire W. This is continued throughout section l3 for the six turns of each wire in each layer.

In section l2, due to the odd number of turns given to wires R and W in passing from section l3 to section l2, the first turn of wire G is over the first turn of wire W and the first turn of wire B is over the first turn of wire R. This arrangement is continued for the six turns of each wire in section I2.

Fig. 9 shows the main or significant capacities that exist between individual turns. Assuming uniformity of conductor diameter and insulation thickness, the capacity between each turn of wires G and R and between each turn of wires B and I W in section l3 and between each turn of wires G and W and between each turn of wires B and R in section l2 will be equal and may be represented by C1. The capacity between each turn of wires G and W and each turn of wires B and R in section I3 and between .each turn of wires G and R and between each turn of wires B and W in section l2 are equal and may be represented by C2.

In Fig. 10 are shown bridge arrangements of capacities C1 and C: for the two sections. For a section of six turns of each individual wire there will be a total capacity of 6C1 between wires R and G and 601 between wires B and W, while between wires -G and W and between wires R and B there will be C2 in each case in section i3. In section l2 there will be 601 between wires R and B and between wires G and W and C2 between wires R and G and between wires B and W.

In telephone practice, side-to-side capacitance unbalance between two conductor pairs is defined in terms of the pairs forming the present device as Therefore, from the bridge arrangement for section I3 the capacitance unbalance is 22C2-l2Cr and for section l2 the capacitance unbalance is l2Cr--22Cz. The two sections are connected in series, thus putting the interwinding capacities in parallel and when these two unbalances 2202- l2C1 and l2Cr-22Cz are added together the result is zero.

For a device of 12 turns of each wire per layer there will be a capacitance unbalance in one section of [2(2n-l)C-22nC1] and in the other section an unbalance of [2 C12(2n 002]. .These unbalances when added together give a resultant unbalance of zero. There are other capacities existant between the turns of the wires but such capacities are so small that they may be disregarded.

The capacity C3 in Fig. 9 of wire R to wire W and C4 of wire G to wire B does not enter into the side-to-side capacitance unbalance but contributes to the capacity loading required to neutralize the inductive reactance added to the transmission line by the insertion of the device of this invention. The capacity loading added by these inherent capacities is normally not suflicient and must be augmented by external capacities. Condensers 32 and 33 of Fig supply this additional loading capacity in connection with the present device.

This invention has been describedin connection with a transmission line in which two cable pairs are shown for the sake of simplicity but the.ln-

,vention is adaptable to many applications and to transmission lines in which many pairs are involved requiring a large number of the inductive devices specifically described. The invention is therefore to be limited only as deilned in the appended claims.

What is claimed is:

-1. An inductive device comprising an insulating tube, a winding of a plurality of turns surrounding one section of said tube, a second winding of a plurality of turns surrounding a second section of said tube, said windings being spaced apart on saidtube a distancesuflicient to reduce to substantially zero the mutual inductance between said windings, a pair of magnetic cores within said tube, one for each winding, said cores being separated from each other by a relatively large non-magnetic gap, and means foradjusting the positions of said cores relatively to said windings.

2. An inductive device comprising an insulating tube, a plurality of windings on said tube, said windings being separated into sections of a plurality of layers, one of said windings comprising non-contiguous turns in one layer of one section, a second of said windings comprising other noncontiguous turns in said one layer of said one section, one turn of said one winding lying in the space between adjacent turns of said second winding, magnetic material in aplurality of concentrated masses within said tube, and means to adjust the position of said masses in relation to said windings.

3. An inductive device comprising an insulating tube, a plurality of windings on said tube, said windings being separated into sections of aplurality of layers, one of said windings comprising non-contiguous turns in one layer of one section, a second of said windings comprising other noncontiguous turns in said one layer of said one section, one turn of said one winding lying in the space between adjacent turns of said second winding, the turns of said one winding and the turns of said second winding being wound in opposite directions,- magnetic material in a plurality of concentrated masses within said tube, and means to adjust the position of said masses in relation to said windings.

4. An inductive device comprising an insulating tube, a plurality of windings on said tube, and means to vary the mutual inductance between said windings while maintaining the inductive reactance of said device substantially constant,

' comprising compressed: magnetic material cores spaced apart and which are adjustable in relation to said windings but fixed in relation to each other.

5. In a transmission system, pairs of conduc tion to said windings, certain of said windings adapted for connection into one of said pairs,

certain others of said windings adapted for connection in another of said pairs, and means to move said magnetic material cores whereby the mutual inductance between said windings may be varied while the inductive impedance of the windings in any one pair remains substantially constant.

6. In a transmission system, pairs of conductors grouped in a cable, an inductive device comprising a plurality of windings, separated cores of compressed finely divided magnetic material movable in relation to said windings, means whereby said windings may be connected to said conductors, and means to move said magnetic material cores to vary the mutual inductance between said windings, whereby voltages in one pair of conductors due to currents in another pair of conductors may be substantially neutraliaed.

7. In a transmission system, pairs of conductors grouped in a cable, an inductive device comprising a plurality of windings, cores of magnetic material spaced from each other and movable in relation to said windings, means whereby said windings may be connected to said conductors and means to move -said magnetic cores whereby the mutual inductance between said windings may be varied while the total inductance in any one conductor of any pair remains substantially constant.

8. An inductive device comprising an insulating tube and a plurality of windings on said tube separated into sections each of a plurality of layers, one of said windings comprising noncontiguous turns in one layer of one section, a second winding comprising non-contiguous turns of one layer of a second section, said non-contiguous turns in said one section and said nonoontiguous turns in said second section being wound on said tube in opposite directions, a third of said windings comprising other non-contiguous turns of said one layer of said one section and a fourth winding comprising other non-contiguous turns of said one layer in said second section, said other non-contiguous turns in said one section and said other non-contiguous turns in said second section being wound in opposite directions.

9. An inductive device comprising an insulating tube and av plurality of windings on said tube separated into sections each of a plurality of layers, one of said windings comprising non-contiguous turns in one layer of one section, a second winding comprising non-contiguous turns of one layer of a second section, said non-contiguous turns in said one section and said non-contiguous turns in said second section being wound on said tube in the same direction, a third of said windings comprising other non-contiguous turns of said one layer of said one section and a fourth winding comprising other non-contiguous turns of said one layer in said second section, said other non-contiguous turns in said one section and said other non-contiguous turns in said second section being wound in the same direction.

10. An inductive device comprising an insulating tube, a plurality of windings on said tube, a plug in the end of said tube, a threaded bushing in said plug approximately centrally thereof, a non-magnetic rod passing into said tube through said bushing, a threaded portion on said rod adapted to coactwith the threads in said bushing, and a plurality of magnetic cores fixed to said rod and separated from each other.

11. An inductive device comprising an insulating tube, a plurality of windings on said tube arranged in sections, a plurality of discontinuous magnetic bodies within said tube fixed relatively to each other but movable relatively to said windings, said windings being grouped in pairs and each winding of each pair being wound to ahelix of substantially the same diameter as that of the other winding of the pair.

12. An inductive device comprising an insulat 13. An inductive device comprising an insulating tube, individual cores within said tube, said cores comprising extremely fine insulated particles of magnetic material pressed together, a

non-magnetic rod supported at both ends in said tube, said cores being fixedly mounted on said rod and separated from each other, a plurality of windings on said tube, each of said windings comprising spaced turns of a single layer, the spaced turns of one winding filling the spaces between the turns of another winding in the same layer.

tions and each section comprising a plurality of layers, each individual winding comprising onehalf the total turns of one layer of one section, the remaining turns of each layer of each section being included in a difierent winding, and a plurality of magnetic bodies associated with said windings.

15. An inductive device comprising an insulating tube, a plurality of windings on said tube, said windings being separated into sections of a plurality of layers, compressed cores of .finely divided and insulated particles of a magnetic material, said cores being separated from each other and fixed in their positions relatively-to each other but movable relatively to said windings, and a metallic shield surrounding said de- 14. An inductive device comprising an insulat ing tube, a plurality of windings on said tube, said windings being located in a plurality of secamazes said tube, other windings mounted on spools whose axes are perpendicular to the axis of said insulating tube, said spools being mounted on said tube, connections from said other windings to certain of said plurality ofwindings to connect said other windings in shunt .to said certain of said plurality of windings, spaced magnetic cores associated with said plurality of windings movable relatively thereto, and ametallic shield surrounding said windings, tube and cores.

17. An inductive device comprising an insulating tube, a plurality of windings on said tube, said windings being separated into sections of a plurality of layers, a plug of insulating material at one end of said tube, a threaded bushing in ,said plug, a non-magnetic rod within said tube, said rod having a threaded portion adapted to coact with said threaded bushing, magnetic material cores on said rod separated from each other but fixed to said rod, elongated terminals of flexible conducting material mounted in holes in said plug and staked to said plug, and a metallic shield surrounding said tube, said windings, said cores, and a part of said plug.

18. An inductive device comprising an insulating tube, grooves cut on said tube in certain parts and in diametrically opposite portions, a plurality of windings on said tube, said windings beingseparated into sections, connections between said windings in said sections, said connections being placed in said grooves, individual magnetic cores in said tube movable relatively to said windings but fixed relatively to each other, a non-magnetic rod joining said cores, and a metallic shield surrounding said windings, said tube and said cores.

19. An inductive device comprising an insulating tube, a plurality of windings on said tube,

said windings being separated into sections of a plurality of layers, each winding comprising onehalf the turns of one layer of one section, separated magnetic cores within said tube movable relatively to said windings, plugs in the ends of said tube, a metallic shield surrounding said tube and windings, and an insulating moisture-repellent compound filling said shield and surrounding. said tube but kept from entering said tu by said plugs.

. PAUL S. DARNELL. 

